The CO4 (and the others in the Cobalt line) are really just incredibly good mikes. I have a total of about 6. They are very "neutral and accurate" sounding. Not much coloring at all, which is what I like. I think the Cobalts are very underappreciated because they are so inexpensive and uncolored. They just do their job, capture the sounds, without generating much quirky snob appeal. After you try one, you will runm back and buy some more, like I did.
There is really not much comparson to an SM57, which is a "classic" mike, but very dull and "muffled" sounding to my ear. I owned 3 or 4 SM57's over te years, but they seemed good only really for recording electric guitar amps. ...just my opinion. I am sure they are used for a lot more.